Laboratory and Field Data Provide a Comprehensive View of
Plutonium Migration Rates at the Nevada National Security
Site. Hydrothermal alteration of nuclear melt glass (inset) yields
colloidal clay with associated Pu. Laboratory colloid formation experiments
along with field measurements reveal the mechanisms of trace
Pu migration from the Benham underground nuclear test. (Note: The
U.S. Environmental Protection Agency regulates Pu in drinking water
at 15 pCi/L.)

All the actinides, which are radioactive, can pose a risk to
human health and the environment. However, plutonium
(Pu) is the most abundant and chemically complex anthropogenic
actinide. Over 2,400 metric tons of Pu are estimated to have
been produced worldwide, with approximately 70–90 metric tons
added to this inventory each year from spent nuclear fuel. A fraction
of this Pu inventory has been released into the environment as a result
of nuclear weapons production, weapons testing, poor waste management,
and nuclear accidents. Subsurface transport of low Pu concentrations
from these environmental releases has been documented on
the scale of kilometers. The large Pu inventory, along with its long
half-life (~24,000 years), toxicity, and known ability to migrate, represents
significant long-term environmental and public health risks.

Neptunium (Np), another long-lived, toxic actinide, is present at
much lower concentrations at contaminated Department of Energy
(DOE) facilities and, thus, represents a lesser environmental
concern. However, both Np and Pu are predicted to be significant
long-term dose contributors in high-level nuclear waste. As a result,
understanding their behavior in the environment is crucial for the
safe, long-term isolation of nuclear waste.

Reliable predictions of how actinides such as Pu and Np will
migrate in the subsurface are not currently possible, preventing
accurate assessments of risk to human health and the environment.
The Subsurface Biogeochemistry of Actinides Scientific Focus
Area (SFA) led by Lawrence Livermore National Laboratory
(LLNL) is addressing this challenge. By identifying the dominant
biogeochemical processes and underlying mechanisms that control
actinide transport (focusing on Pu and Np), the SFA is advancing
efforts to reliably predict and control actinide cycling and mobility
in the subsurface. The project is supported by DOE’s Office of
Biological and Environmental Research (BER), within DOE’s
Office of Science, as part of BER’s Subsurface Biogeochemical
Research (SBR) program.

Key Knowledge Gaps

The LLNL SFA is focused on advancing understanding of subsurface
actinide behavior to provide a scientific basis for remediation and
long-term stewardship of DOE legacy sites and, more broadly, increase
understanding of transport phenomena in environmental system science.
Key knowledge gaps addressed by the LLNL SFA include:

Mechanisms driving surface-mediated Pu and Np reduction.

Formation of stable natural organic matter coatings on mineral
surfaces and their effect on Pu and Np redox transformations and
sorption reactions.

Mechanisms responsible for observed Pu transport behavior in the field.

Subsurface Biogeochemistry of Actinides. The LLNL SFA is
building an understanding of actinide behavior from the atomic to
field scale. Findings from these observations across scales are used to
develop conceptual and numerical models and improve prediction of
actinide migration at globally relevant sites.

Linking Laboratory and Field Observations

Predicting actinide behavior in the environment necessitates a
scaled approach that integrates laboratory experiments and computational
models with field observations of actinide transport. Computational
models (e.g., quantum mechanical calculations) reveal
the fundamental structure of actinide complexes at the atomic level.
Laboratory experiments provide quantitative data on the affinities,
kinetics, and morphology of actinide associations with mineral surfaces,
organic matter, and microbes. Field observations provide the
foundation for conceptual understanding of actinide migration.

This integrated laboratory- and field-scale approach to subsurface
biogeochemistry of actinides is being used to quantify actinide
transport at contaminated sites such as the Nevada National Security
Site (NNSS), Hanford Site (Richland, Washington), Savannah River Site (Aiken, South Carolina), and Sellafield (Cumbria, England). At
NNSS, for example, detailed adsorption and desorption experiments,
hydrothermal colloid formation experiments, and field actinide
groundwater analyses suggest that Pu will migrate in groundwater
for decades. However, actinide concentrations are likely to decline
over time and unlikely to reach hazardous levels. Such SFA findings
are providing the scientific foundation for understanding actinide
transport phenomena and applying scientifically defensible
management decisions at actinide-contaminated sites worldwide.

Computational Models Determine Actinide Behavior Under
Spectroscopically Inaccessible Conditions. Snapshots of the
first coordination shell for [Th(OH)n](4–n)+(aq) as obtained from first-principles
molecular dynamics computer models provide insights into
the coordination chemistry of Pu and Np. For clarity, only thorium (Th)
and the first shell of oxygens are shown. Th is purple, hydroxyl oxygen
(OH) is red, and water oxygen (OH2) is blue. The fully hydrolyzed
complex Th(OH)4(aq) (left) is seven-coordinate, with four hydroxyls
and three waters in the first shell. Successive removal of hydroxyls
(left to right) initially leads to a substitution by water to maintain
the seven-coordinate structure. Upon removal of the third hydroxyl
(right), however, the coordination number increases to eight, and the
structure changes to a square antiprism.

Nanoscale Characterization Tools Identify Pu Associations with
Contaminated Sediments. (Left) Scanning electron microscopy
reveals the presence of plagioclase and other minerals in contaminated
sediments recovered 25 m beneath the Z-9 trench at the Hanford Site in
Richland, Washington. (Right) NanoSIMS is used to identify the spatial
distribution of silicon (Si), aluminum (Al), iron (Fe), and plutonium (Pu) in
these same grains and indicates a strong correlation between Pu and Fe.

Highlights

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